270 Pleasant Ways in Science. 



him. just what smaller numbers do to the savage — a quantity 

 greater than he can adequately comprehend. 



Astronomy brings us into acquaintance with motions of 

 enormous velocity traversing prodigious spaces, chemistry and 

 physics exhibit motions equally wonderful for their incalculable 

 speed, and yet performed in spaces too minute for us to con- 

 ceive. A swallow in its swiftest flight is said to move at the 

 rate of ninety miles an hour ; Mercury, in its journey round the 

 sun, performs about 109,360 miles in an hour;* Venus, being 

 less near the centre of our system, is contented with a march 

 of 80,000 miles an hour ; while our earth, in the same time, 

 traverses 68,040 miles. These are wonderfully quick motions, 

 but they are nothing to the velocity of light, which travels, 

 according to recent experiments, at the rate of about 185,000 

 miles in a second.f 



A great quantity of evidence leads to the conclusion that 

 light is a motion of an extremely subtle fluid, as sound is the 

 motion of air. J Now, wave motion is somewhat complicated. 

 If two persons hold a long rope, and one lifts his end rapidly 

 up and down, a wave motion visibly passes from one end to 

 the other. A portion of the rope rises and falls, passing its 

 motion on to the next portion, until the whole rope is affected 

 by a beautiful series of wave lines. Now the velocity with 

 which each portion rises and falls is one thing, and the velocity 

 with which the wave form is transmitted from one part to another 

 is another thing ; and when wave actions take place, there may 

 be an enormous difference between these two velocities. When 

 we speak of light coming to us from sun or star with a rapidity 

 of 185,000 miles a second, we mean that the waveform reaches 

 us with that degree of speed ; but the velocity with which the 

 ether particles rise and fall in their vibrations is infinitely more 

 swift. When we investigate sounds, we find their pitch de- 

 pends upon the slowness or rapidity of their motions — a 

 metallic strip that vibrates with great rapidity giving an acute 

 sound, which becomes graver as the velocity of vibration is 

 diminished. A complete vibration is the oscillation of a par- 

 ticle once forwards and once backwards through a greater or 

 smaller arc. The intensity of a sound depends on the size or 

 amplitude of the vibration, and its pitch upon their velocity. 



* These figures are taken from Sir J. Herschel's Outlines of Astronomy. 



f This Telocity of light corresponds -with the distance now assigned to the 

 sun upon astronomical grounds — namely, 91,600,000 miles. 



J It must not be understood that sound is exclusively produced by vibrations 

 of common air. In Gemot' s Physics, translated by Atkinson, we find it well stated 

 that " sound is a peculiar sensation excited in the organ of heaving by the vibra- 

 tory motion of bodies, when this motion is transmitted to the ear through an 

 elastic medium." Solids and fluids will conduct sounds, and so will all gase. j . 

 The sounds we commonly hear and make use of are vibrations of atmospheric 

 air. 



